In recent years, radio frequency identification (RFID) has gained a lot of publicity. An important advantage of RFID is its ability to identify multiple objects or tags wirelessly without a direct line of sight between the objects and the reader device. Furthermore, the versatility of RFID tags in terms of their shapes, sizes, ranges and types makes them superior to conventional bar codes.
RFID is a system for tracking and identifying multiple objects in a reader's interrogation zone simultaneously via magnetic or electromagnetic response exchange. Such a system typically consists of an RFID reader device and a finite number of RFID tags. The tags contain miniaturized integrated circuits with a small antenna for communicating with the reader device. Usually, identification information can be stored in the integrated circuits. RFID tags can be active, passive or semi-passive. Passive tags have no power source: they use the power emitted by the reader device to energize and transmit their identification codes to the reader device. Semi-passive and active tags have an on-board power source, and are activated by a reader's field. Active tags do not require the reader to be present in order to operate; they have an on-board transmitter for sending data or identification codes. Passive and semi-passive tags are cheap compared to active tags, and are therefore more suitable for large-scale deployments.
In the technical field of identification systems, tags are also referred to as transponders. A reader device is also referred to as a base station. The data or identification codes are, for example, the serial numbers of transponders in a magnetic field.
An important aspect of RFID systems is the use of so-called anti-collision protocols. Tag collisions arise when multiple tags respond simultaneously to a reader's request. FIG. 1 illustrates a prior art identification system wherein a reader R receives, via an antenna A, simultaneous responses from multiple transponders T1-T4. This results in collisions at the reader, which leads to bandwidth and energy wastage, and a prolonged tag identification time.
Many anti-collision protocols have been designed in order to resolve and avoid collisions. The anti-collision protocols according to the prior art typically deploy schemes wherein a dedicated active time slot is randomly selected for transmission of the data or identification codes. The use of these dedicated active time slots increases the complexity of the slot allocations and therefore the overall cost.